Epoxy resins are well known in the art. In combination with a suitable curing agent, they result in thermosetting products showing superior toughness, chemical resistance, heat resistance, adhesion and electrical properties.
The most common types of epoxy resins are those which are based on bisphenol-A and which contain 1,2-epoxy groups. These compounds can be made by reaction of bisphenol-A with epichlorohydrin. The reaction is often carried out in such a way that liquid reaction products are obtained, but higher molecular weight semi-solid and solid products are also produced in this way. Another process to produce higher molecular weight semi-solid and solid resinous polyepoxides is a process known as "upgrading" or "advancement". In such an upgrading or advancement process, usually an initially liquid resinous polyepoxide is reacted with a dihydric phenol in the presence of a catalyst until the required amount of the dihydric phenol is incorporated in the epoxy chain to increase the molecular weight to the desired level.
It will be appreciated that said upgrading or advancement process as described hereinbefore with reference to dihydric phenol, can also be carried out using carboxyl compound or other hydroxyl compounds.
Such upgrading processes have been described in the past both on a batch basis and on a continuous basis, see e.g. U.S. Pat. Nos. 3,547,881, 3,919,169 and 4,105,634. In such known batch and continuous processes, the dihydric phenol and liquid epoxy resin are mixed together at a relatively low temperature and then heated up to the reaction temperature and held at elevated temperature for the time sufficient to produce the resinous epoxy compound of the higher molecular weight. The catalyst is usually added either to the starting reaction mixture at the relatively low temperature or after heating of the reacting mixture to the reaction temperature.
In such known batch and continuous upgrading processes, however, cycle times, including dumping, are typically relatively lengthy. For example, batch processes involving bisphenol-A and a liquid polyepoxide consisting essentially of the diglycidyl ether of bisphenol-A can take from 4 to 20 hours for the reaction to be completed. Further, the homogeneity of temperature in a large kettle reactor is complicated by heat transfer, i.e. the heat of reaction is more difficult to control and localized high heats will cause adverse reactions to occur, e.g. crosslinking and/or gelling. Furthermore, the reaction may continue during dumping when the conditions are less controlled, resulting in different conditions for different batches and thus in different product properties for each batch. The continuous process using a pipe reactor described in U.S. Pat. No. 3,919,169 involves a shorter reaction time in the order of about two hours, but in a continuous process it would be highly advantageous if the reaction time could be significantly lower. Also, due to the flow profile in a pipe, the use of pipe reactors often results in a rather broad molecular weight distribution and in a fouling of the reactor wall, ultimately resulting in a thick layer of deposited material which needs regular cleaning.
In addition to economies of time, long reaction times can lead to a relatively wide molecular weight distribution which may in their turn lead to end use disadvantages. For example, surface coating imperfections due to gel particles (MEK-insolubles, MEK is methyl ethyl ketone) have been observed when molecular weight distribution and concomitant viscosity characteristics are not properly controlled.
Another possibility to produce advanced epoxy resin is the use of an extruder process. However, there are numerous disadvantages to such a process. The investment costs are relatively high, the more because high performance extruders are needed (to avoid any stagnant zones). Further, a relatively high effort is needed to operate the extruder, maintenance is cumbersome and the product quality is not optimal, especially due to gel formation (gel particles which do not dissolve in MEK). These gel particles may result in a low quality cured product. Another disadvantage is the relatively short residence time in an extruder (typically up to five minutes). This short residence time results in the need to use relatively high catalyst concentration to complete the reaction, as illustrated in U.S. Pat. No. 4,612,156.
In addition it is observed that, depending on the intended use of the advanced resin, in some cases the upgrading process is preferably carried out in the absence of a solvent in order to avoid solvent stripping and vacuum devolatilization since even then the final product contains significant amounts of undesirable solvents. These residual solvents may cause numerous problems when the product is fabricated into a usable product, such as films, by coextrusion or moulding. The residual solvents require extensive vacuum drying to prevent voids in the film and moulded articles. The hazard of solvents being released from a product during fabrication could cause a problem unless proper venting is employed. Solvents may have an adverse effect on polymer properties such as stability, colour, haze etc.
It will be appreciated that depending on the relative amounts of epoxy compound and the compound having at least one hydroxyl compound or carboxyl compound, the end product is epoxy compound or a hydroxyl or carboxyl compound. The above described problems, however, hold for all end products.